The present invention relates to radio frequency identification circuitry, and more particularly, to transceiver architectures for radio frequency identification systems.
Radio frequency identification (RFID) systems are wireless telemetry systems, whereby a transponder or RFID “tag” communicates information to an interrogation transceiver or RFID “reader.” The information provided by the RFID tag may be location information, the tag's identity, or other data. Continued improvements in circuit miniaturization have resulted in very small RFID tags that can be unobtrusively placed on an item which is to provide the aforementioned telemetry. The RFID tag communicates the information to the RFID reader when interrogated, or otherwise activated to do so.
FIG. 1 illustrates a simplified block diagram of an RFID system. The system includes an RFID reader 110 and one or more RFID tags 120, between which RFID signals 102 are communicated. The RFID reader 110 operates as a transceiver whereby it transmits an interrogation signal 102a, and receives signal responses 102b from one or more of the RFID tags. The RFID reader 110 is shown with a single transmit/receive antenna, but may include different transmit and receive antennas (or more than one of each) to more effectively communicate with each of the RFID tags 120. The RFID reader 110 may be fixed, or handheld to permit mobile scanning and interrogation of an item. The RFID tags 120 may be battery-powered or passive, in which case the RFID tag obtains power from the transmitted signal 102 for its operation. The RFID tag 120 includes an antenna that may be in the printed form to maintain the tag's small footprint. The system may employ frequency or signal polarization diversity or anti-collision protocols to permit greater communication ability.
A central component of the RFID system is the transceiver or RFID reader 110. Conventionally, the receiver portion of the RFID reader 110 has been designed using either a homodyne mixing approach or a superheterodyne mixing approach. The homodyne mixing approach provides advantages in that the circuit architecture is relatively simple and generally power efficient. The difficulties with such a direct conversion receiver are well known and include dynamically varying DC offset level, I/Q mismatch, and erroneous baseband signals generated by the cross-modulation of the RF signal (i.e., the received RFID signal) and the LO signal (the internally generated reference signal).
The superheterodyne mixing approach provides some relief to the aforementioned obstacles, but at significant expense. The superheterodyne design provides relatively good image rejection using an image rejection mixer and corresponding frequency synthesizer. The disadvantage of such a system is that it creates a requirement for an additional frequency synthesizer, as the transmitting and receiving channels typically require their own frequency source in this design. The presence of both frequency synthesizers decreases available chip real estate and drains much needed power.
Accordingly, what is needed is an improved transceiver architecture which provides good image rejection through the implementation of fewer circuit components.